Optical disk with marks used for calibrating an optical detector to minimize noise from undesired perturbations in disk surfaces

ABSTRACT

A magnetooptical disk recording device has a far-field detector for detecting radial position of a laser beam with respect to tracks on the disk. The far-field detector has a reference line dividing first and second photo detectors. The reference line is centered radially with respect to center lines of the tracks. The reference line is also center lengthwise to be tangentially center with respect to a tangential point on the track. The detector is adjustable both radially and tangentially. The tangential adjustment is based upon sensing two low reflective calibration marks on the disk that indicate centering the laser beam, hence the track tangent point, on the reference line. The calibration is based upon obtaining minimum noise in the detector output signal that signifies that the laser beam is centered on the length of the detector reference line. The calibration marks are in a mirror area disposed at one radial extremity of the disk. The marks are radially elongate and disposed in a radially outward diverge with respect to each other. The marks are disposed at 45 degrees with respect to two circumferentially spaced apart radial lines of the disk.

This application is a continuation of application Ser. No. 07/960,033,filed Oct. 13, 1992, now abandoned.

FIELD OF THE INVENTION

This invention pertains to optical disk drives, more particularly toapparatus and methods related to adjustment of optical detectors forminimizing noise in detector generated signals caused by contaminants orother undesired disk surface perturbations.

BACKGROUND OF THE INVENTION

Optical disk drives employ a multi-element detector for generating aso-called track error signal (TES) that in other servo positioningsystems is termed a position error signal. TES indicates relativeposition of a focused laser beam with respect to a center of a track, inmost present day optical disks such track is a spiral groove formed in adisk substrate. One detector for generating TES is a far field detectorthat has two photo responsive elements. A line between the two elementsis aligned tangentially with the spiral track (groove) and is preferablycentered on the track center line. Such centering provides an accurateTES. The far field detector elements supply their respective signals toa differential amplifier that outputs a differential signal termed TES.TES can also be on two lines, one line connected to each of theelements. The signal amplitude difference of the signals on the twolines are a push-pull position error signal.

Tolerances for positioning a detector in an optical disk drive arerelative to the size of the reflected laser beam being detected. Inso-called near field (such as astigmatic detectors) detector thereflected laser beam is focussed to a small cross-sectional area makingpositional tolerances small. In contrast, a far field detector thereflected laser beam is focussed to have a larger cross-sectional areasuch that positioning tolerances for the detector are relatively large,such as an order of magnitude greater than in near field detectors. Suchincreased tolerances reduces sensitivity of the detector output signal,such as a track error signal, to thermal changes.

Also, to obtain a small size optical subassembly or head for an opticaldevice, it is desired to use detectors that occupy small areas. Thisdesire dictates that the photo element size in a detector have a smallerarea than the cross-section of a reflected defocussed laser beam.

A problem in precise positioning of the focussed laser beam on a micronwide optical data-storing track or in precise optical track seekingoperations arises from undesired surface contamination or perturbationsthat change reflectivity of the disk surface. Such disk surfaceperturbations can be in the millimeter and sub-millimeter size and yetcause sufficient noise in a tracking error signal (TES) to reduce diskdrive performance. Such undesired changes in reflectivity (i.e. reducedreflectivity) have been found to cause a pair of areas of reducedreflected light intensity (shadows) in a far field light pattern of areflected laser beam. Such far field light is used for optical disktracking and seeking. It is believed that similar problems arise in nearfield light pattern of a reflected laser beam. One of the "shadows" isgenerated as the contaminant blocks or partially blocks some of thelaser beam as the beam impinges on the contaminant as the laser beamenters the disk. A second "shadow" is generated by the reflected lightbeam traveling from the recording surface exiting the disk to return tothe objective lens and the known detectors. The locations of the shadowsremain balanced in the reflected laser beam as it travels through theobjective lens into the optical system for detection. The shadows arecentered about the optical axis of the reflected light beam. It has beendiscovered that if the far field detector that generates TES is notcentered on the reflected light beam in a direction tangential to theoptical track (groove) on the disk, then the shadows are imposed on thetwo photoelements of the far field detector asymmetrically or resultingin TES erroneous amplitudes. It is desired to overcome the above statedproblem in a simple and effective manner.

DISCUSSION OF THE PRIOR ART

The U.S. Pat. No. 5,036,505 issued to Gleim shows readback circuits thatadjust their operation to compensate for undesired surfaceperturbations, such as contaminants, on an optical disk. In contrast,the present invention provides for an adjustment of the trackingdetector that centers a reflected beam on the tracking detector forcompensating for noise induced into a tracking error signal TES byundesired surface perturbations.

The Takei et al Japanese published patent document 62-239330, publishedOct. 20, 1987, based on application 61-81514 shows a read back circuithaving a gate that opens and closes for generating a focus error signalin the presence of disk surface induced noise. Again, it is desired toprovide an adjustment to a detector for minimizing signal noise inducedby undesired surface perturbations.

Uejima in Japanese patent document 61-8746, published Jan. 16, 1986,shows spreading or distorting the reflected laser beam in a directiontangential to the track being followed, i.e. transversely to the lineseparating two photo elements of a tracking error detector. According tothis patent document, the beam distortion reduces effects of signalnoise induced by disk surface contaminations. In contrast, the presentinvention does not distort the reflected light beam for maintainingtracking accuracy, rather, the detector is adjusted tangentially tominimize signal noise effects caused by undesired surface perturbations.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a detection system inan optical apparatus wherein a laser beam is centered radially andtangentially on a multielement detector for minimizing signal noisedcaused by scanning undesired surface perturbations, such ascontaminants.

In accordance with the present invention an optical laser beam detectorof an optical disk drive is symmetrically disposed with respect to thecenter of a track to be scanned by a laser beam and centered about atangent point on such data storage track.

In accordance with one particular aspect of this invention, a trackingerror detector is adjusted tangentially to minimize effects of undesiredsignal noise induced by surface perturbations and contaminants. A diskhaving low reflectance marks acting as pseudo contaminants, preferablyin a so-called mirror area of an optical disk, are disposed on a diskusable with an optical disk drive. The laser beam of the optical deviceis caused to repeatedly traverse the pseudo contaminants. During suchtraversals, the detector tangential position is adjusted until theamplitude of the contaminant induced signal noise is minimum. Then thedetector is secured in the adjusted position. In this aspect of theinvention, the detector is adjustable both radially and tangentiallywith respect to data tracks on the disk.

In another aspect of the present invention, an optical disk drive has anoptical detector tangentially adjusted to minimize signal noise fromundesired surface perturbations. In yet another aspect of the invention,an optical disk having pseudo contaminants outside a recording area isprovided for enabling adjusting detectors in an optical device forminimizing signal noise caused by undesired surface perturbations.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an optical disk drive using the present invention ina simplified block-schematic diagram.

FIG. 2 diagrammatically illustrates an optical disk usable in the FIG. 1illustrated drive and that incorporates one aspect the presentinvention.

FIG. 3 is a simplified diagrammatic plan view of a subassemblyconstructed in accordance with an aspect of the present invention andusable in the FIG. 1 illustrated drive.

FIG's. 4 and 5 are flow charts showing initial and post-manufacturingchecking and adjustment of the FIG. 3 illustrated subassembly in a driveconstructed as shown in FIG. 1.

FIG. 6 diagrammatically illustrates a partial cross-section of anoptical disk for showing surface contaminates and a structure used inadjusting the FIG. 1 illustrated drive in accordance with the presentinvention.

DETAILED DESCRIPTION

Referring now more particularly to the appended drawing, like numeralsindicate like parts and structural features in the various figures. FIG.1 illustrates a magnetooptical disk drive that incorporates the presentinvention. A magnetooptic record disk 30 is mounted for rotation onspindle 31 by motor 32. Optical portion 33 is mounted on frame 35. Aheadarm carriage 34 moves radially of disk 30 for carrying an objectivelens 45 from track to track. A frame 35 of recorder suitably mountscarriage 34 for reciprocating radial motions. The radial motions ofcarriage 34 enable access to any one of a plurality of concentric tracksor circumvolutions of a spiral track, preferably identified by a spiralgroove in the disk 30 surface, for recording and recovering data on andfrom the disk. Linear actuator 36 suitably mounted on frame 35, radiallymoves carriage 34 for enabling track accessing. The recorder is suitablyattached to one or more host processors 37. Such host processors may becontrol units, personal computers, large system computers, communicationsystems, image signal processors, and the like. Attaching circuits 38provide the logical and electrical connections between the opticalrecorder and the attaching host processors 37.

Microprocessor 40 controls the recorder including the attachment to thehost processor 37. Control data, status data, commands and the like areexchanged between attaching circuits 38 and microprocessor 40 viabidirectional bus 43. Included in microprocessor 40 is a program ormicrocode-storing, read-only memory (ROM) 41 and a data and controlsignal storing random-access memory (RAM) 42.

The optics of the recorder include an objective or focusing lens 45mounted for focusing and radial tracking motions on carriage 34 by fineactuator 46. Actuator 46 includes mechanisms for moving lens 45 towardand away from disk 30 for focusing and for radial movements parallel tocarriage 34 motions; for example, for changing tracks within a range of100 tracks so that carriage 34 need not be actuated each time a trackadjacent to a track currently being accessed is to be accessed. Numeral47 denotes a two-way light path between lens 45 and disk 30.

In magnetooptic recording, magnetic bias field generating coil 48 duringdata writing or recording supplies a bias or steering magnetic field todisk 30 for directing remanent magnetization to the direction of thefield. In a constructed embodiment electromagnet provides a weakmagnetic steering or bias field for directing the remnant magnetizationdirection of a small spot on disk 30 illuminated by laser light fromlens 45. The laser light spot heats the illuminated spot on the recorddisk to a temperature above the Curie point of the magnetooptic layer(not shown, but can be an alloy of rare earth and transitional metals astaught by Chaudhari et al., U.S. Pat. No. 3,949,387). This heatingenables magnet coil 48 generated bias field to direct the remnantmagnetization to a desired direction of magnetization as the spot coolsbelow the Curie point temperature. Magnet coil 48 is shown as supplyinga bias field oriented in the "write" direction, i.e., binary onesrecorded on disk 30 normally are "north pole remnant magnetization". Toerase disk 30, magnet coil 48 supplies a field so the south pole isadjacent disk 30. Magnet coil 48 control 49 is electrically coupled tomagnet coil 48 over line 50 to control the write and erase directions ofthe coil 48 generated magnetic field. Microprocessor 40 supplies controlsignals over line 51 to control 49 for effecting reversal of the biasfield magnetic polarity.

It is necessary to control the radial, transversely to a track, positionof the beam following path 47 such that a track or circumvolution isfaithfully followed longitudinally along its length for scanning thelaser beam on path 47 along the length of the track. Also, preciselycontrolling the radial position enables a desired track orcircumvolution to be quickly and precisely accessed. To this end, focusand tracking circuits 54 control both the coarse actuator 36 and fineactuator 46. The positioning of carriage 34 by actuator 36 is preciselycontrolled by control signals supplied by circuits 54 over line 55 toactuator 36. Additionally, the fine actuator 46 control by circuits 54is exercised through control signals travelling to fine actuator 46 overlines 57 and 58, respectively for effecting respective focus and trackfollowing and seeking actions. Sensor 56 senses the relative position offine actuator 46 to headarm carriage 34 to create a relative positionerror (RPE) signal supplied to focus and tracking circuit 54 over line119.

The focus position sensing is achieved by analyzing laser lightreflected from disk 30 over path 47, thence through lens 45, throughpartially-polarizing beam splitter 60 and to be reflected bypartially-polarizing beam splitter 61 to a so-called "quad detector" 62.Quad detector 62 has four photoelements which respectively supplysignals on four lines collectively denominated by numeral 63F to focusand tracking circuits 54. Focusing operations are achieved by comparingthe light intensities detected by the four photoelements in the quaddetector 62. Tracking and seeking control is provided through beamsplitter 73 directing a portion of the reflected laser light receivedfrom splitter 61 to split-photo or far-field detector 74 that is mountedon circuit board 76 adjustably supported by optical unit 33, as willbecome apparent. Detector 74 supplies the tracking error signal TES overline 63T to focus and tracking circuits 54. Aligning one axis of thedetector 74 with a track center line, track following operations areenabled. Track following and seeking are performed in a usual manner.

In accordance with the present invention, detector 74 is positionedradially (transversely to the track) and tangentially (substantiallyalong the track length, i.e. at a tangent to the circumvolution of thetrack) such that detector 74 center line 112 (FIG. 3) is tangential tothe center line of the spiral track of disk 30 and the center of thelaser beam is centered along the length of line 112. That is, the centerof the reflected laser beam is centered on detector 74 in both theradial and tangential directions. Focus and tracking circuits 54 analyzethe signals on lines 63F and 63T to respectively control focus andtracking.

Recording or writing data onto disk 30 is next described. It is assumedthat magnetic field from coil 48 is switched to the desired polarity forrecording data. Microprocessor 40 supplies a control signal over line 65to laser control 66 for indicating that a recording operation is toensue. This means that laser 67 is energized by control 66 to emit ahigh-intensity laser light beam for recording; in contrast, for reading,the laser 67 emitted laser light beam is a reduced intensity for notheating the laser illuminated spot on disk 30 above the Curie point.Control 66 supplies its control signal over line 68 to laser 67 andreceives a feedback signal over line 69 indicating the laser 67 emittedlight intensity. Control 68 adjusts the light intensity to the desiredvalue. Laser 67, a semiconductor laser, such as a gallium-arsenide diodelaser, can be modulated by data signals so the emitted light beamrepresents the data to be recorded by intensity modulation. In thisregard, data circuits 75 (later described) supply data indicatingsignals over line 78 to laser 67 for effecting such modulation. Thismodulated light beam passes through collimating lens 71 toward halfmirror 60 for being reflected toward disk 30 through lens 45. Datacircuits 75 are prepared for recording by the microprocessor 40supplying suitable control signals over line 76. Microprocessor 40 inpreparing circuits 75 is responding to commands for recording receivedfrom a host processor 37 via attaching circuits 38. Once data circuits75 are prepared, data is transferred directly between host processor 37and data circuits 75 through attaching circuits 38. Data circuits 75have ancillary circuits (not shown) relating to disk 30 format signals,error detection and correction and the like. Circuits 75, during a reador recovery action, strip the format signals and other non-data signalsfrom the readback signals before supplying corrected data signals overbus 77 to host processor 37 via attachment circuits 38.

Reading or recovering data from disk 30 for transmission to a hostprocessor requires optical and electrical processing of the laser lightbeam from the disk 30. That portion of the reflected light (which hasits linear polarization from laser 67 rotated by disk 30 during readbackusing the Kerr effect) travels along the two-way light path 47 andthrough lens 45 and partially-polarizing beamsplitters 60 and 61 to thedata detection portion 79 of the headarm 33 optics. Half waveplate 64,through suitable orientation of its fast axis, rotates the incidentlinear polarization through an angle of 45 degrees. Polarizingbeamsplitter 80 transmits all light of one linear polarizationorientation while reflecting all light of the orthogonal polarizationorientation in such a way that when no Kerr effect is present, the powerin both the transmitted and reflected beams are equal. The reflected andtransmitted beams are focussed by lenses 81 and 83 onto photocells 82and 94, respectively. Differential amplifier 85 subtracts the outputsignal amplitudes supplied by photocells 82 and 84. When the reflectedlight is rotated by a "south" or erased pole direction remnantmagnetization, the output of photocell 84 is slightly greater than theoutput of photocell 82 causing the differential amplifier 35 to create asignal of a certain polarity. When the reflected light is rotated in theopposite direction by a "north" or written pole direction, the output ofphotocell 82 is slightly greater than the output of photocell 84 causingthe differential amplifier 85 to create a signal of the oppositepolarity. This is termed "differential magneto-optic detection," andenables good common-mode rejection of variations in light intensitywhich contain no signal information. The amplifier 85 supplies theresulting difference signal (data representing) to data circuits 75 fordetection. The detected signals include not only data that is recordedbut also all of the so-called ancillary signals as well. The term "data"as used herein is intended to include any and all information-bearingsignals, preferably of the digital discrete value type.

The rotational position and rotational speed of spindle 31 is sensed bya suitable tachometer or emitter sensor 90. Sensor 90, preferably of theoptical-sensing type that senses dark and light spots on a tachometerwheel (not shown) of spindle 31, supplies the "tach" signals (digitalsignals) to RPS circuit 91 which detects the rotational position ofspindle 31 and supplies rotational information-bearing signals tomicroprocessor 40. Microprocessor 40 employs such rotational signals forcontrolling access to data storing segments on disk 30 as is widelypracticed in the magnetic data storing disks. Additionally, the sensor90 signals also travel to spindle speed control circuits 93 forcontrolling motor 32 to rotate spindle 31 at a constant rotationalspeed. Control 93 may include a crystal-controlled oscillator forcontrolling motor 32 speed, as is well known. Microprocessor 40 suppliescontrol signals over line 94 to control 93 in the usual manner.

An optical disk constructed in accordance with the present invention isshown in FIG. 2. Such optical disk may be used solely as a calibrationdisk or may be a usual data storing disk having adjustment enablingpseudo contaminants in one of two mirror areas. The mirror areas arenon-recording areas. No grooves are in the mirror areas, i.e. the mirrorarea is a planar reflecting light reflecting area. Disk 30 rotates aboutaxis 99 via mounting hub 100. An inner diameter (ID) mirror area 101(shown with a greater radial dimensions than desired for a practicalembodiment) is disposed radially between hub 100 and annular datarecording area 102. A spiral groove in area 102 is coated withmagnetooptical material for forming a machine sensible data storingtrack, as is known. The outer diameter mirror area 103 is disposedradially between the disk periphery and annular recording area 102. Theillustrated radial dimension of area 103 is enlarged while the radialdimension of area 102 is shown reduced for better illustrating theinvention.

In an early embodiment of the invention pseudo contaminant markers 104were affixed to the surface of disk 30 in area 103 substantially asshown. Markers 104 simulate contaminants for causing two shadows thatare caused by a contaminant or other undesired surface perturbation thatreduces reflectivity of the disk. A radial inward end of the twoelongated markers are circumferentially spaced apart. Also, theelongated markers 104 were disposed at about 45° from two respectiveradial lines at predetermined locations in the markers 104, such as atthe radially inward ends of the two markers. The circumferential edgesof markers 104 cause two shadows in the reflected laser beam to simulatea contaminant, as will as other undesired surface perturbations. Thelength of markers 104 is best empirically determined in that the lengthof the markers 104 are longer than the diameter of a defocussed laserbeam such that the entire beam always scans the markers, as laterdescribed. As an example, the width of each pair of markers 104 wasapproximately one millimeter with a length of several millimeters, nolimit thereto intended. The circumferential width of markers 104 is notcritical.

Markers 105, constructed similarly to markers 104, are similarlydisposed in inner diameter mirror area 101. Either or both sets ofmarkers 104 and 105 may be used to control tangentially adjustingdetector 74 as will become apparent. In practicing the preferredembodiment of the invention, laser beam travelling over path 47 toeither mirror area is focussed on the mirror region or area 101/103(also area 165 of FIG. 6) resulting in an out-of-focus beam at theoutwardly facing surface 166 (FIG. 6) of the media substrate 161 (FIG.6) with a cross-section diameter in the millimeter range. In thepreferred embodiment the lengths of each marker 104 or 105 is greaterthan the cross-sectional diameter of the laser beam as it shines ontoeither mirror area 101 or 103. Therefore, each marker 104, 105 has alength along the radius of the disk of more than one millimeter. Thewidth (circumferential extent) of each marker is not critical. Thedescription of FIGS. 4 and 5 indicate the reasons for the abovestatements.

To cause the laser beam to scan markers 104, the coarse actuatorcarriage 34 is first positioned at an outer radial position and lockedin that position, i.e. carriage 34 will not move radially. Similarly forscanning markers 105 with the laser beam, carriage is locked in positionat a radially inward position. The fine actuator 46 is locked in focuson the media mirror surface 165 (FIG. 6) by focus and tracking circuits54. The radial position of fine actuator is locked at a center positionof its radial range for maintaining the laser beam at one radialposition. This fixed positioning avoids requiring any trackfollowing--i.e. there are no grooves in the mirror area to follow.Therefore, the laser beam is radially positioned to always scan themarkers 104/105 respectively in the outer or inner mirror areas. Assuch, the laser beam scans the markers once each disk rotation. Further,since the markers in each pair of markers 104/105 are disposed at 45°with respect to two radial lines, the marker induced noise has both apositive and negative excursion giving an indication of direction ofmisadjustment of detector 74 that results in a minimal noise effect ontrack following.

FIG. 3 illustrates detector subassembly circuit board 76 in a simplifieddiagrammatic inverted plan view. Board 76 is adjustably mounted formovement radially of disk 30 and in a transverse direction tangential tothe optical disk track (not shown). The two-way adjustment enablesprecisely positioning detector 74 such that the reflected laser beam iscentered on detector 74 both radially and tangentially. Detector 74 ismounted on board 76 such that its center line 112 between its two photosensors (not numbered) is aligned with the radial direction of disk 30,as represented in FIG. 3 by double-headed arrow 110. Double-headed arrow111 represents the tangential direction of the spiral track (groove) ofdisk 30, as is known. Flat flexible cable 115 attached to circuit board76 includes line(s) 63T to carry the detected track position errorsignal from detector 74, as processed in integrated circuit module 113(shown as dashed line as circuit module 113 is on a surface of board 76that faces away from the illustrated surface supporting detector 74. Theelectrical circuit connections are not shown in FIG. 3. A pair of flatflexible cables 116 also extend from board 76 to other circuits of theFIG. 1 illustrated device, such as for providing electrical power toboard 76. Optics 33 are mounted on frame 35 as an assembly. Board 76, inturn, is adjustably mounted on the assembly frame (not separately shownbut represented by the box enclosing the optical elements of optics 33)of optics 33 as described below.

The radial adjustment of detector 74 is achieved by moving board 76 inthe directions of arrow 110 for moving board 76 radially with respect tothe track center line. To this end, three elongated slots 120-122 areformed in board 76 in a manner to provide a triangle. In the prior artthe width of slots 120-122 closely approximated the diameter offasteners 124-126, respectively, for minimizing tangential skewing ofboard 76 during radial alignment of line 112 with a track center line.Fasteners 124-126 include a head that has an extent greater than thewidth of elongated slots 120-122. Such fasteners may be self-tappingscrews, bolts with lock nuts, and the like.

Elongated slots 120-122 have a width substantially greater than thediameter of fasteners 124-126. This additional width enables movingcircuit board 76, hence detector 74, along the center line 112, i.e.tangentially to the track center line. Of course, the heads (not shown)on the fasteners 124-126 have an extent to maintain gripping contactwith board 76 for each and every tangential adjustment position.

For facilitating manual and automatic adjustment of board 76, datum orreference holes 130-131 are provided. A tool (not shown) is insertedinto holes 130-131 for accomplishing both the radial and tangentialadjustments. The details of the tool are not important to reaching anunderstanding of the present invention.

Referring next to FIG. 4, initial adjustment of detector 74 in the FIG.1 illustrated device is briefly described. Step 135 mounts circuit board76 on the optics 33 frame (i.e. head frame). A tool is suitably insertedinto datum holes 130 and 131 for precisely moving board 76 with respectto the head frame. In a manual adjustment, the signal on line 63T issupplied to an oscilloscope. An operator then moves board 76 with a handtool for achieving the below described results. In an automatic system,the line 63T signal is supplied to an analyzer (not shown) that in turncontrols a robot (not shown) for moving board 76 as below described. Thedesign of either a manual system or automatic system is well within thecapabilities of one of ordinary skill for practicing the presentinvention.

After board 76 is mounted (or loosened if already installed) preparatoryto the initial adjustment, carriage 34 is moved radially to a maximalradially outward position and kept locked in one radial position. Fineactuator 46 is locked at its center position(not shown) of its range ofradial motion. Objective lens 45 is focussed on the mirror surface 165(FIG. 6) resulting in an out-of-focus beam condition at the outersurface 166 of the media substrate 161 such that the beam cross sectionat the outer surface 166 has a diameter on the order of a millimeter.

At step 136, board 76 is adjusted radially along the lengths of theelongated slots 120-122 in a known manner. After centering line 112 at atrack center line, the tool maintains the radial position. At steps137-139 the tangential adjustment is achieved that centers the reflectedlaser beam on detector 75 in the tangential direction. The adjustmentmay require several steps, i.e. several executions of the process loop137-139 for finding the lowest amplitude of TES as the laser beam inpath 47 circumferentially traverses marks 104/105 while tracking andfollowing circuits 54 are in the focus mode. Upon finding the minimumTES noise signal (min) at step 139, then board 76, hence detector 74 aresecured at that position.

In a similar manner, after the FIG. 1 illustrated device is in thefield, FIG. 5 illustrates the procedures for verifying the initialadjustment of FIG. 4 or for tangentially adjusting board 76 on anoptical drive that is not so adjusted. Step 145 measures the TES noiseamplitude from marks 104/105. If the sensed amplitude is withinpredetermined specifications (empirically determined), then from branchstep 146 the procedure is exited for indicating a drive that has beenadjusted in accordance with this invention. Otherwise, if thespecifications are not met, then step 150 loosens fasteners 124-126 forpermitting adjustment of board 76 position. Steps 151-153 perform thefunctions described for steps 137-139 of FIG. 4. Upon obtaining aminimum noise signal amplitude, step 154 secures board 76 in theadjusted position.

FIG. 6 diagrammatically illustrates a partial radial cross-section ofdisk 30. Disk 30 includes a recording layer 160, such as ofmagnetooptical material supported on transparent substrate 161. Therecording area of disk 30 includes a spiral groove represented byrectangle 162 that constitutes a grooved region of disk 30 ending atcircumferential line 164. Such grooved area 162 typically has a spiralgroove of a radial extent in the micron range. Mirror region having aflat reflective surface 165 facing the outer annular surface 166 ofsubstrate 161 is usually designated as not being used for recordingdata. According to this invention, annular mirror area 165 is used toadjust detector 74 tangential position to minimize noise effects ofsurface perturbations/contaminants 167 that may occur on outwardlyfacing annular surface 166. Markers 104 are suitably secured tooutwardly facing surface 166 in a superposed relation to mirror surface165, such as also seen in FIG. 2. As shown in FIG. 2, markers 104 and105 can be respectively at a radially outward or inward of recording orgrooved area 162, denoted by numeral 102 in FIG. 2. Therefore, the FIG.6 showing is generic to using either a radially inward or outward mirrorarea 101 or 103 of FIG. 2. The laser beam on path 47 (FIG. 1) has across-section at outwardly-facing surface 166 while being focused to apredetermined focus condition at mirror surface 165. Contaminant orperturbation 167 can be in the millimeter or sub-millimeter size, asmentioned above.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. An optical disk having an axis of rotation and apredetermined annular outwardly-facing surface area on a transparentsubstrate and with an annular reflective layer disposed in the disk onthe substrate facing and spaced axially inwardly from theoutwardly-facing surface area, a plurality of substantially concentricdata-storing tracks on the disk, each of said tracks having apredetermined radial extent; comprising:said reflective layer having apredetermined high optical reflectance, said outwardly-facing surfacearea being subject to having predetermined undesired shadow-causingsurface perturbations that interfere with a light beam impinging uponand being reflected from said annular reflective layer; low reflectancemeans on a predetermined portion of the surface area for simulating saidpredetermined undesired surface perturbations and having a reflectanceless than said predetermined high optical reflectance; said reflectivelayer having substantially circular groove means therein thatconstitutes said tracks, each of said tracks having a radial extent notgreater than five microns, an annular non-grooved mirror area in thereflective layer, said non-grooved mirror area having a radial extentgreater than said radial length of said low reflectance means and whollycontaining said predetermined area such that the low reflectance meansis disposed entirely on said non-groove mirror area; and said lowreflectance means including a pair of elongated opaque marks, each ofsaid opaque marks having a radial length at least an order of magnitudegreater than said radial extent of each of said tracks, said marks beingcircumferentially spaced-apart, disposed at a first angle with respectto each other and respectively disposed at an acute angle with respectto a radial line disposed between said marks, each said acute anglebeing substantially one-half of said first angle.
 2. The optical diskset forth in claim 1, further comprising:said acute angle beingsubstantially 45 degrees.
 3. The optical disk set forth in claim 1,further comprising:said annular non-grooved mirror area being disposedat one radial extremity of said reflective layer.